Phase change heat sink for transient thermal management

BACKGROUND OF THE INVENTION

0001Heat producing devices, such as printed circuit boards, often contain heat producing components, such as processors or voltage regulators. A thermal plane may be provided in combination with the heat producing devices to form an assembly to aid in the removal of heat, typically by providing additional conductive pathways to disperse the heat. Typically, air cooling and liquid cooling systems are used in open environments where the heat may be dissipated to the surroundings. In certain instances, the heat producing components may operate under transient modes of increased heat production where high heat dissipations are expected over short durations. The transient mode may exceed the capacity of the cooling system, unless the cooling system is sized for the worst-case transient response, which results in over capacity for steady-state operation.

BRIEF DESCRIPTION OF THE INVENTION

0002In one aspect, a heat dissipating assembly includes at least one heat producing component and a heat sink having phase change material conductively coupled to the at least one heat producing component, wherein the phase change material changes between at least two phases in response to the conductive transfer of heat from the heat producing component to the phase change material.

BRIEF DESCRIPTION OF THE DRAWINGS

0003In the drawings:

0004<figref idref="DRAWINGS">FIG. 1</figref> is a schematic cross sectional view of a printed circuit board assembly where a heat producing component is in indirect conductive contact with the heat sink according to one embodiment of the invention.

0005<figref idref="DRAWINGS">FIG. 2</figref> is a schematic cross sectional view taken along line <b>2</b>-<b>2</b> of <figref idref="DRAWINGS">FIG. 1</figref> showing the heat sink.

0006<figref idref="DRAWINGS">FIG. 3</figref> is a schematic cross sectional view of a printed circuit board assembly where a heat producing component is in direct conductive contact with the heat sink according to a second embodiment of the invention.

0007<figref idref="DRAWINGS">FIG. 4</figref> is a schematic cross sectional view of a printed circuit board assembly showing an alternate heat producing component placement.

0008<figref idref="DRAWINGS">FIG. 5</figref> is a schematic cross sectional of a chassis assembly where a heat producing component is in indirect conductive contact with the heat sink according to one embodiment of the invention.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

0009The embodiments of the present invention are related to a heat dissipating assembly comprising at least one heat producing component. In the embodiment of <figref idref="DRAWINGS">FIG. 1</figref>, a printed circuit board (PCB) assembly <b>10</b> is shown comprising a PCB, exemplified as a pulse laser control board (PLCB) <b>12</b> to operate a pulse laser apparatus (not shown), having heat producing components <b>14</b>, shown as microprocessors, on the PLCB top surface <b>16</b>. Pulse lasers are known to operate at high power requirements, causing high heat production in the PCB components, for short periods of time (on the order of seconds). Although microprocessors are illustrated, additional heat producing components <b>14</b>, such as power regulators, resistors, inductors, capacitors, etc., may be provided on the PCB.

0010The PCB assembly <b>10</b> further comprises thermal planes <b>24</b>, <b>26</b> having a heat sink <b>20</b>, and thermal pads <b>22</b>, conductively coupling the heat producing component to at least one of the thermal planes <b>24</b>, <b>26</b>. The thermal planes are shown as an upper thermal plane <b>24</b> and a lower thermal plane <b>26</b>, each at least partially encompassing the top and bottom surfaces <b>16</b>, <b>18</b> of the PLCB <b>12</b>, respectively, and configured to mount to the PLCB <b>12</b>. The thermal planes <b>24</b>, <b>26</b> are illustrated as aluminum, which is efficient at conducting and dissipating heat. Alternately, the thermal planes <b>24</b>, <b>26</b> may comprise any material able to efficiently conduct or dissipate heat. Although both an upper thermal plane <b>24</b> and lower thermal plane <b>26</b> are shown, further embodiments may have only a single thermal plane in proximity to the at least one heat producing component, or proximate to a heat dissipation need, as design considerations require.

0011The PCB assembly <b>10</b> further defines a fastening component that is provided to aid in the mounting of the upper thermal plane <b>24</b> and the lower thermal plane <b>26</b> to the PLCB <b>12</b>. As illustrated, the fastening component comprises a fastener, shown as a screw <b>28</b>, received through an opening <b>30</b> in the upper thermal plane <b>24</b>, PLCB <b>12</b>, and lower thermal plane <b>26</b>. Any suitable fastening component may be used. For example, other mechanical fasteners, e.g. bolts, nails, pins, etc., may be used as well as non-mechanical fasteners, such as welding or adhesive. Alternatively, the screw <b>28</b> may couple the upper and lower thermal planes <b>24</b>, <b>26</b> directly to each other, wherein the PLCB <b>12</b> will be partially or entirely contained within the planes <b>24</b>, <b>26</b>.

0012The thermal planes may further be fastened using a screw-on clamp at two opposing side ends <b>29</b>, <b>31</b> of each upper and lower thermal planes <b>24</b>, <b>26</b>. The upper thermal plane <b>24</b> is clamped to the lower thermal plane <b>26</b> to obtain a low compressive force (e.g. 0.023 to 0.069 m-kg) on to the PLCB <b>12</b> components.

0013In a sense, these clamps provide for a clamshell mount of the upper and lower thermal planes <b>24</b>, <b>26</b> about the PLCB <b>12</b>. The coupling of the upper and lower thermal planes <b>24</b>, <b>26</b>, regardless of the manner of coupling, results in a housing structure that contains the PLCB <b>12</b> in a manner similar to a clamshell. While not necessary, it is contemplated for the upper and lower thermal planes <b>24</b>, <b>26</b> to be hinged along one edge to have a true clamshell configuration. However, as used in this description, the term clamshell is not limited to a hinged coupling. This clamshell arrangement allows for ease of assembly and disassembly during maintenance of the PLCB <b>12</b>.

0014The thermal pads <b>22</b>, illustrated as conductive putty, provide for physical contact and reliable thermal conduction between the heat producing component <b>14</b> and the upper thermal plane <b>24</b>, as shown. Alternate embodiments of thermal pads may include thermal paste, or adhesive-type materials with suitable conductive properties.

0015Turning now to <figref idref="DRAWINGS">FIG. 2</figref>, the details of the heat sink <b>20</b> will be described. The heat sink <b>20</b> is defined by an inner wall <b>32</b> and includes a conductive frame <b>34</b> defining a plurality of chambers <b>35</b> in which the phase change material <b>36</b> is received. As illustrated, the frame <b>34</b> is illustrated as a grid of interconnected walls, which happen to define a honeycomb cross section, but other cross sections are contemplated. However, other types of frame structures, both grid and non-grid, are contemplated. The frame <b>34</b> enhances the conduction of heat into and away from the phase change material <b>36</b> within the heat sink <b>20</b>. The inner wall <b>32</b> is exemplified as an elastomer material for retaining the phase change material <b>36</b>, but may be any type of material suitable for heat conduction.

0016The frame <b>34</b> shown operates to segregate the phase change material in separate chambers <b>35</b>, while providing an interconnected heat conduction path from the upper thermal plane <b>24</b> to the inner geometric chambers <b>35</b> of the phase change material. Other materials, for example, copper, steel, nickel, or copper tungsten, etc., may provide similar structural and heat conductive properties functionality of frame <b>34</b>, and provide design flexibility to incorporate or address concerns such as electromagnetic interference (EMI) protection, weigh, or thermal expansion/contraction. This aforementioned list of grid materials should not be considered exhaustive, but rather, examples of a wide range of materials suitable for particular applications.

0017The phase change material <b>36</b> is exemplified as paraffin wax (CaCl₂*6H₂O), a solid which changes phase to a liquid when sufficient heat is absorbed, and further changes phase from a liquid back to a solid when sufficient heat is released. A typical melting point of paraffin wax is between 46 and 68 degrees Celsius. After the phase change from solid to liquid, the paraffin wax is able to further absorb supplementary heat, beyond the heat causing the phase change to occur. The paraffin wax is configured to repeatedly change phase from a solid to a liquid, and back to a solid, reliably for multiple cycles.

0018Other phase change materials, for example ammonia, are envisioned, so long as the material changes phase from a starting phase to at least one different phase in response to the conductive transfer of heat. The starting phase and at least one different phase may be one of solid, liquid, and gas. In the current embodiment, the paraffin wax may also include an additive material, such as carbon black, to increase the radiation heat transfer emissivity.

0019During operation of the pulse laser, the heat producing component <b>14</b> on the PLCB <b>12</b> uses electrical power over short durations (on the order of seconds), creating a mode of high power consumption interspersed with modes of low power consumption, and as a result, the need for transient high heat dissipation during the high power consumption modes. During these transient durations, the heat produced by the heat producing component <b>14</b> is transferred by conduction to the thermal pad <b>22</b>, followed by the upper and lower thermal planes <b>24</b>, <b>26</b>. The heat is then transferred to the heat sink <b>20</b> by the inner wall <b>32</b>, with the frame <b>34</b> conducting the heat to the phase change material <b>36</b>, where the heat is absorbed. In this sense, the phase change material <b>36</b> provides a physical material for heat storage during heat dissipation of the PCB during the transient duration. Upon the completion of the transient duration, the PCB assembly <b>10</b> will then shed the heat stored in the phase change material by conduction from the phase change material back to the thermal plane, via the frame <b>34</b>, to the surrounding environment over time through radiation.

0020During exceptionally high heat or a prolonged transient heat condition, the phase change material <b>36</b> will melt, changing phase from a solid to a liquid or a liquid to a gas. This change of phase allows the phase change material <b>36</b> to absorb additional heat during the high heat transient cycle for later dissipation. In such a scenario, the liquid phase change material <b>36</b>, upon subsequent heat release occurring during the low heat portion of the operation cycle, will phase change back to a solid.

0021By way of non-limiting example, in a typical example of a low power, greater duration heat dissipation condition, a 0.127 m by 0.1778 m PCB assembly comprising a thermal plane, and a heat sink with a honeycomb structure having 0.0051 m thickness of paraffin wax, said PCB assembly is able to maintain a constant temperature of a 10.7 Watt heat load for 30 minutes without the need for external cooling.

0022Additional heat dissipation devices may be used in conjunction with the exemplified embodiment, such as air cooling fins or liquid cooling channels, to provide for increased heat relief Moreover, the entire PCB assembly <b>10</b> structure may be coated or painted with a black, high emissivity, low gloss material to ensure effective radiation heat transfer to the surrounding environment.

0023The above-mentioned embodiment is an example of the heat sink <b>20</b> configured in indirect conductive contact with the heat producing component <b>14</b> (via the thermal pad <b>22</b> and thermal planes <b>24</b>, <b>26</b>). Alternatively, it is envisioned that the heat sink <b>20</b> may be configured to be in direct conductive contact with at least one heat producing component wherein the heat sink <b>20</b> and heat producing component have a common platform for heat transfer.

0024<figref idref="DRAWINGS">FIG. 3</figref> illustrates an alternative PCB assembly <b>110</b> according to a second embodiment of the invention. The second embodiment is similar to the first embodiment; therefore, like parts will be identified with like numerals increased by <b>100</b>, with it being understood that the description of the like parts of the first embodiment applies to the second embodiment, unless otherwise noted. A difference between the first embodiment and the second embodiment, as illustrated, is that the upper thermal plane <b>124</b> is configured to receive only a single heat producing component, placing the heat sink <b>120</b> in direct conductive contact with the heat producing component <b>114</b>, instead of full PLCB <b>112</b> coverage and indirect conductive contact of the first embodiment. As illustrated, the second embodiment further lacks the thermal pad of the first embodiment.

0025In this embodiment, the upper thermal layer <b>124</b> and heat sink <b>120</b> are configured or formed with protrusions and ridges with precision tolerance to mate directly over the heat producing component <b>114</b>.

0026Many other possible embodiments and configurations in addition to that shown in the above figures are contemplated by the present disclosure. For example, one embodiment of the invention contemplates the thermal planes <b>24</b>, <b>26</b> having indirect or direct conductive contact with the full PLCB surfaces <b>16</b>, <b>18</b> or only a portion of the PLCB surfaces <b>16</b>, <b>18</b>. Likewise, the thermal planes <b>24</b>, <b>26</b> may have indirect or direct conductive contact with the full heat producing components <b>14</b> surface, or only a portion of the heat producing component <b>14</b> surfaces. In direct contact configurations, the thermal pad would not be needed. Although a thermal pad or similar material may be provided to ensure complete contact with the thermal plane and the heat producing component.

0027While the heat sink <b>20</b> is shown at least partially embedded within each of the thermal planes <b>24</b>, <b>26</b>, other configurations are contemplated. The heat sink <b>20</b> may be completely embedded within one or both of the thermal planes <b>24</b>, <b>26</b>. The heat sink <b>20</b> may reside on one or both of an upper or lower surface of one or both of the thermal planes <b>24</b>, <b>26</b>. The heat sink <b>20</b> may be integrally formed with at least one of the thermal planes <b>24</b>, <b>26</b>. Alternatively, the heat sink <b>20</b> may be mounted to at least one of the thermal planes <b>24</b>, <b>26</b>.

0028Additionally, the design and placement of the various components may be rearranged such that a number of different configurations could be realized. For example, <figref idref="DRAWINGS">FIG. 4</figref> illustrates alternate assemblies of the PLCB <b>212</b>, including placement of heat producing components <b>214</b>, such as microprocessors on the PLCB bottom surface <b>218</b>, or even integrated within the PLCB <b>212</b>, are encompassed by the one embodiment of the invention. As shown, the heat sinks <b>220</b> are in indirect contact with each heat producing component <b>214</b>, via a lower thermal plane <b>226</b>.

0029Furthermore, non-PCB assemblies are included. For example, <figref idref="DRAWINGS">FIG. 5</figref> illustrates a different type of assembly that requires heat dissipation. In <figref idref="DRAWINGS">FIG. 5</figref>, a pulse radar chassis assembly <b>340</b> is shown, comprising a heat producing component <b>314</b>, a chassis <b>338</b>, and heat sinks <b>320</b>. While a pulse radar chassis assembly <b>340</b> is shown, any chassis having a heat producing component that requires heat dissipation, such as a high power microwave chassis, a radio transmitting chassis, etc., is envisioned. Although the pulse radar chassis assembly <b>340</b> is shown having heat sinks <b>320</b> in indirect contact with the heat producing component <b>314</b>, alternate configurations, as described herein, are envisioned.

0030It is contemplated that any of the embodiments may be combined. For example, the embodiments of <figref idref="DRAWINGS">FIGS. 1 and 4</figref> may be combined to locate the heat producing components on both the upper and lower surface of the PCB. One or more of these heat producing elements may have an individual heat sink according to the embodiment of <figref idref="DRAWINGS">FIG. 3</figref>, while the remaining heat producing elements are connected to the heat sink according to the embodiment of <figref idref="DRAWINGS">FIG. 1</figref>. It is further contemplated that the heat sinks of embodiments of <figref idref="DRAWINGS">FIGS. 1 and 3</figref> may be combined resulting in “stacked’ heat sinks.

0031The embodiments disclosed herein provide a PCB assembly having a heat sink with phase change material. One advantage that may be realized in the above embodiments is that the above described embodiments have superior weight and size advantages over the conventional type PCB assemblies having air cooling fins or liquid cooling components. With the proposed phase change material heat sink arrangement, a high heat dissipation can be achieved during transient heat conditions without additional heat dissipation elements since the reliable heat absorption of the phase change material is inherent, providing heat storage with the physical material for later release during lower heat production conditions.

0032Moreover, higher PCB reliability can be achieved even when components do not have high heat transient conditions because the phase change material provides exceptional heat dissipation properties during steady state operation or reduced environmental cooling conditions, such as changes to environmental temperature under high solar settings. Additionally, one aspect of the invention lowers the required enthalpy of a liquid or air heat exchanger needed to cool electronics during transient thermal cycles since the enthalpy is not designed for transient loads, but rather steady loads.

0033When designing PCB assemblies, important factors to address are size, weight, and reliability. The above described PCB assemblies have a decreased number of parts and less electrical draw compared to a PCB assembly having air or liquid cooling, making the complete system inherently more reliable. This results in a lower weight, smaller sized, increased performance, and increased reliability system. The lower number of parts and reduced maintenance will lead to a lower product costs and lower operating costs. Reduced weight and size correlate to competitive advantages.

0034This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.